The present invention is generally directed to an ion conductor or electrolyte for galvanic elements, including lithium batteries with solid or at least immobilized electrolytes containing powdery, chemically inert solids.
In conventional secondary batteries such as lead storage batteries or in alkaline batteries, steps have been taken to limit mobility within the aqueous solutions of sulfuric acid or alkali liquor, for reasons of reliability. In the case of lead-acid electrolytes, for example, this is achieved by determining the necessary minimum amount of electrolyte to be retained in the separator, or gel. As always, the more energy-rich cells, with alkali metal anodes, contain solvent-based liquid electrolytes. Electrolyte salts that are frequently used for this purpose include sodium or lithium salts with univalent anions such as BF.sub.4 --, AlCl.sub.4 --, PF.sub.6 --, AsF.sub.6 --, ClO.sub.4 --, or CF.sub.3 SO.sub.3 --, which are dissolved in solvents such as propylene carbonate, acetonitrile, gamma-butyrolactone or methyltetrahydrofuran. These electrolytes have a specific conductivity (.kappa.) between 10.sup.-3 and 10.sup.-2 S/cm at room temperature (see D. Linden, "Handbook of Batteries & Fuel Cells", McGraw-Hill, 1984).
However, the structure of the more reliable solid batteries requires a solid, or at least a pasty electrolyte system. To date, the polyether complexes of alkali salts of the general structure poly-[(ethylene oxide).sub.n M .sup.+ X .sup.- ], developed as the most positive representatives of a solid ion conductor (see, e.g., R. Huq and G. C. Farrington, Solid State Ionics 28/30, 990 (1988)), have achieved specific conductivity .kappa. values of only about 10.sup.-5 S/cm at room temperature. Such values are too low (by two to three orders of magnitude for practical application.
Various modifications of these polymer ion conductors have been proposed in recent publications, but have also not produced practical (useful) specific conductivities combined with thermal or chemical stability (see, e.g., J. L. Bennet et al., Chem. Materials 1, 14 (1989) or D. Fish et al., Brit Polym. J. 20, 281 (1988)). For example, the polyethylene oxides tend to decompose either thermally or in the presence of reactive substituents into dioxane or other products. Additional attempts to provide solid ion conductors which are suitable as electrolytes or separators for solid batteries can be found in the patent literature.
For example, U.S. Pat. Nos. 2,933,547 and 2,861,116 disclose phenol-resin-based ion-exchange membranes which contain Zn.sup.2+ ions for the mobile phase which are used as electrolytes for galvanic solid-state elements with zinc anodes. U.S. Pat. No. 3,186,876 and United Kingdom Patent No. 999,948 disclose a pure Na zeolite (e.g., in a Cu/Zn element) functioning as an ion-conducting separator, and in addition to this, a Cu.sup.2+ zeolite which functions as a catholyte produced by a partial exchange of Na.sup.+ ions.
European patent application 070,020 discloses a solid proton conductor used as an electrolyte, which is preferably also a zeolite. The disclosed proton conductivity is due to the insertion of proton-containing cations (e.g., H.sub.3 O.sup.+, NH.sub.4.sup.+, N.sub.2 H.sub.5.sup.+) into the crystal structure, in which the immobile since they are bound to oxygen atoms of the disclosed silicic acid framework while forming hydroxyl groups. In this case, a "basic phase" (NH.sub.3, H.sub.2 O, alcohol, organic amine) is introduced into the zeolite lattice to promote the transport of ions. Proton conductivities of more than 10.sup.-3 S/cm at room temperature, which are useful for battery purposes, were found with this material.